When you see them all together
(Fig 1) in the family tree of Astrapotherium you’ll notice that ‘Hapsburg chin‘ – an underbite that goes back at least to Meniscotherium.
Very few vertebrates have an underbite.
Figure 1. Astrapotherium ancestors back to Late Paleocene Orthaspidotherium.
According to Wikipedia – Astrapotheria,
‘Astrapotheres were large, rhinoceros-like animals and have been called one of the most bizarre orders of mammals with an enigmatic evolutionary history. This taxonomy of this order is not clear.”
In the LRT several taxa (Fig 1) are the relatives and ancestors of Astrapotherium.
The present results
of the large reptile tree (LRT, 2319 taxa) currently required confirmation, refutation or modification with a similar taxon list and a suitably long list of multistate characters (i.e. 200+, as in the LRT).
Orthaspidotherium edwardsi
(Ladeveze et al. 2010; Late Paleocene, 58 mya, 7.5cm skull length) nests with Meniscotherium. The skull is twice as wide as tall on this ?weasel-like terrestrial mammal. The jawline is convex at the canine and beneath the orbit. The lacrimal/jugal/zygomatic arch is quite robust.
Meniscotherium tapiacitum
(Cope 1874; Williamson and Lucas 1992; Middle Eocene 54-38 mya; 25-50 cm long) nests with Astrapotherium (below) in a clade near the base of the Condylarthra. Considering the Paleocene date for Astrapotherium, Meniscotherium probably originated earlier, perhaps in the Cretaceous. The canines became larger and larger until they became pig-like tusks in this clade.
Figure 3. Periphragnis reconstructed from an in situ skeletal mount. Note the wide flat unguals.
Periphragnis exauctus
(Roth 1899 (genus), Ameghino 1902 (species), Middle Eocene to Early Oligocene; 1.5m long), is traditionally considered very close to the marsupial Thomashuxleya. At this point Periphragnis nests with the Astrapotherium clade among the condylarth placentals. It had five hoof-like unguals on all four extremities, distinct from otherwise similar marsupials with only four digits.
Trigonostylops wortmani
(Ameghino 1897; Late Paleocene—Early Eocene; est. 1.5m) nests between Meniscotherium and Brachycurus in the LRT, basal to other astrapotheres.
Brachycrus laticeps (originally Merycochoerus Douglass 1900; CM796; Miocene, 14mya; 1m long) was considered an North American oreodont with a tapir-like trunk and deep jaws, but here nests with South American Astrapotherium.Both are derived from North American Meniscotherium
Astraponotus assymemetrum
(Ameghino 1901; Late Eocene 35mya) is a transitional South American taxon nesting with Astropotherium, midway in size between it and Brachycurus.
Astrapotherium magnum
(Burmeister 1879, Hatcher 1901; Paleocene-Miocene, 59-12 mya; 3m long) was considered a South American ungulate. Like a hippo, the large and ever-growing curved canines of Astrapotherium scraped against each other during life producing sharp tips. No longer uniquely, the rostrum was much shorter than the mandibles. The feet and toes were all small. The narial opening was elevated to the top of the skull. Astrapotherium likely had a tapir-like trunk.
References
Ameghino F 1902. Cuadro sinoptico de las formaciones sedimentarias terciarias y cretacicas de la Argentina, en relacion con el desarrollo y descendencia de los mamiferos. An. Mus. Nac. Hist. Nat., vol. 8, pp. 1-12.
Ameghino F 1897. Les mamiferes crétacés de l´Argentine. Boletín Instituto Geográfico Argentino:18: 405–521.
Ameghino F 1901. Notices préliminaires sur des ongulés nouveaux des terrains crétacés de Patagonie [Preliminary notes on new ungulates from the Cretaceous terrains of Patagonia]. Boletin de la Academia Nacional de Ciencias de Córdoba 16:349-429.
Burmeister 1879. Description physique de al République Agentine, T. III 1879:517.
Cooper LN, Seiffert ER, Clementz M, Madar SI, Bajpai S, Hussain ST, Thewissen JGM 2014-10-08. Anthracobunids from the Middle Eocene of India and Pakistan Are Stem Perissodactyls. PLoS ONE. 9 (10): e109232. doi:10.1371/journal.pone.0109232. PMID 25295875.
Cope ED 1872. Descriptions of some new Vertebrata from the Bridger Group of the Eocene. Proceedings of the American Philosophical Society 12:460-465.
Cope ED 1874. Report of the Chief of Engineers for. 1874 Vol. II, Pt. II, Appendix FFa of Appendix FF, pp. 115-130.
Cope ED 1881. Notes on Creodonta. American Naturalist 15: 1018–1020.
Douglass E 1900. New species of Merycochoerus in Montana. Part I. American Journal of Science 10(60):428-438.
Geisler J and McKenna MC 2007. A new species of mesonychian mammal from the lower Eocene of Mongolia and its phylogenetic relationships. Acta Palaeontologica Polonica 52, 189-212.
Hatcher JB 1901. Report of the Princeton University Expeditions to Patagonia 1869-1899. Mammalia of the Santa Cruz Beds. IV. Astrapotheria. Scott WB ed. Vol. 6, Paleontology 3. Princeton, NJ Stuttgart 1909-1928.
MacPhee RDW, et al. (5 co-authors) 2021. Cranial Morphology and Phylogenetic Relationships of Trigonostylops wortmani, an Eocene South American Native Ungulate. Bulletin of the American Museum of Natural History 449(1), 1-183. https://doi.org/10.1206/0003-0090.449.1.1
Ladevèze S, Missiaen P and Smith T 2010. First Skull Of Orthaspidotherium Edwardsi (Mammalia, “Condylarthra”) From The Late Paleocene Of Berru (France) And Phylogenetic Affinities Of The Enigmatic European Family Pleuraspidotheriidae. Journal of Vertebrate Paleontology 30(5): 1559–1578.
Roth S 1899. Aviso preliminar sobre mamíferos mesozoicos encontrados en Patagonia. Revista del Museo de La Plata 9: 381–388.
wiki/Meniscotherium
wiki/Dissacus
wki/Orthaspidotherium
wiki/Periphragnis
wki/Astrapotherium
wiki/Brachycrus
wiki/Astraponotus
wiki/Trigonostylops
wiki/Astrapotheria
The Pterosaur Heresies 